Standards for vehicle fuel systems evaporative emissions are becoming stricter, and the required test procedures are changing, as well. Testing entails keeping the vehicle within an enclosure, generally called a test shed, and measuring the evaporative fuel emissions therefrom over a set time. This is done by measuring the concentration of evaporated fuel in the air within the enclosure. Knowing that, and the volume of air in the shed, the mass of fuel emission can be calculated. Such calculation fails if there is an air exchange or leak across the shed wall during the test. Unless there is a significant pressure differential between the interior of the shed and ambient, however, such exchange is minimal. Still, it is very difficult and expensive to build an absolutely leakproof enclosure.
A new test regimen will require that the temperature inside the shed be sent significantly above and below the ambient during the test cycle, so as to better simulate the conditions that a fuel system would see over an average day. Of course, increasing and decreasing the temperature inside an enclosure will increase and decrease the air pressure. In addition, the length of the test, some 72 hours, means that barometric pressure differentials between the ambient and the enclosure will be experienced. A pressure differential will act across any potential leak paths without some compensation. One proposal is to build a shed with a roof that will rise and fall with the temperature, increasing and decreasing the shed volume so as to prevent the pressure change. While workable, this would be very expensive. Another proposal is to leave the shed volume constant, but to keep the pressure constant by withdrawing air from the main enclosure into a separate enclosure as the temperature rises, and vice versa. This inevitably withdraws some of the fuel vapor emissions from the shed, which have to be separately measured at the end of the test.